any of a group of chemical agents released by neurons (neuron) (nerve cells) to stimulate neighbouring neurons, thus allowing impulses to be passed from one cell to the next throughout the nervous system.

The site where neurons meet is called the synapse and consists of the axon terminal (transmitting end) of one cell and the dendrite (receiving end) of the next. A microscopic gap called a synaptic cleft exists between the neurons. When a nerve impulse arrives at the axon terminal of one neuron, a chemical substance is released through the presynaptic membrane, traveling in milliseconds across the synaptic cleft to the postsynaptic membrane of the adjoining neuron. The chemical release is stimulated by the electrical activity of the neuron.

Although a large number of chemical substances are believed to act as neurotransmitters, only a few have been identified. Among those known are acetylcholine, norepinephrine (noradrenalin), dopamine, and serotonin. Some chemical agents, such as acetylcholine, activate neurons, while others act as inhibiting substances.